(1) Field of the Invention
The present invention relates to a so-called "tandem-type" image forming apparatus, and particularly relates to a technique for adjusting variations in sensitivity among sensors when the sensors respectively detect toner images formed by image forming units to control an image forming condition.
(2) Related Art
For conventional color copiers, a method using a transfer drum or intermediate transfer component has been commonly employed. A color copier using this method is provided with a single image forming unit, and a photosensitive drum in the image forming unit is rotated for each color. Toner images formed corresponding to colors cyan, magenta, yellow, and black are superimposed to form a color image. Hereinafter, these toner colors are referred to as "C", "M", "TY", and "K".
As stated above, this conventional color copier is provided with a single photosensitive drum. Thus, for the automatic image density control (referred to as the "AIDC" hereinafter) and the correction, the amount of toner attracted to the photosensitive drum is detected for each color using a single photoelectric sensor (referred to as the "AIDC sensor" hereinafter).
The following is a brief description of an example of the AIDC performed using the AIDC sensor.
For the AIDC, the AIDC sensor detects the density of a standard patch (referred to as the "AIDC pattern" hereinafter). In accordance with the detection value, a grid voltage VG of a sensitizing charger and a developing bias voltage VB of a developing unit are controlled by referring to a table that is stored in a memory beforehand. Consequently, the image density is appropriately maintained.
FIG. 1 is a schematic diagram showing an arrangement of a photosensitive drum 301, a sensitizing charger 302, and a developing unit 303 of the image forming unit provided in the conventional color copier. As shown in this diagram, the sensitizing charger 302 is set facing the photosensitive drum 301, with the discharge voltage being referred to as VC. A grid 3021 of the sensitizing charger 302 is applied a negative grid voltage VG by a VG generating unit 304. The grid voltage VG is considered to be almost equivalent to a surface potential VO of the photosensitive drum 1C. This is to say, the surface potential VO of the photosensitive drum 301 can be controlled by adjusting the grid voltage VG.
A developing roller 3031 of the developing unit 303 is applied a negative developing bias voltage VB by a VB generating unit 305, with the developing bias voltage VB satisfying an inequality .vertline.VB.vertline.&lt;.vertline.VO.vertline.. Accordingly, the surface potential of the developing roller 3031 becomes VB.
When the laser exposure is performed on the photosensitive drum 301 in this state, the surface potential of the exposed part on the photosensitive drum 301 increases and becomes a voltage VI. When the voltage VI becomes higher than the developing bias voltage VB, negatively charged toner carried to the surface of the developing sleeve of the developing roller 3031 is attracted to the exposed part on the photosensitive drum 301. The higher a developing voltage .DELTA.V is, the more amount of toner is attracted to the exposed part. Here, the developing voltage .DELTA.V is calculated according to the following equation. EQU .DELTA.V=.vertline.VB-VI.vertline.
As such, the developing voltage .DELTA.V changes as the surface potential VO and the bias voltage VB are changed.
Consequently, the amount of toner attracted to the photosensitive drum 301 can be controlled by changing the surface potential VO and the bias voltage VB.
With this being the situation, a AIDC sensor 306 detects the amount of toner in the AIDC pattern that is formed on the photosensitive drum 301 under a predetermined level of exposure. In accordance with the detection result, the VG and the VB are adjusted, so that the density of the AIDC pattern can be maintained constant.
More specifically, the values of the VG and VB are set from the detection value obtained by the AIDC sensor 306 in accordance with a table shown in FIG. 2 which is experimentally obtained.
In recent years, tandem-type color copier have been increasingly used. In a tandem-type color copier, toner images respectively formed on photosensitive drums set along a transfer belt are transferred onto a recording sheet transported on the transfer belt or onto the transfer belt to form a color image.
In this tandem-type color copier, an image forming unit is provided for each color. Thus, it is preferable to provide an AIDC sensor for each image forming unit to detect the amount of attracted toner. By doing so, the AIDC can be separately performed for each color.
In this case, however, if the sensitivity of the AIDC sensors varies, a color balance of the formed image is inappropriate.
When the amount of attracted toner is detected by the single AIDC sensor as explained about the conventional color copier employing the method using the transfer drum, it is a minor problem if a detection value obtained by the AIDC sensor has an error. In this case, the detected density for each color is darker or lighter than an actual color at the same level based on the error. Therefore, each density of toner images is adjusted in the same direction in accordance with the error, so that the color balance can be maintained.
Meanwhile, when the AIDC sensor for detecting the amount of toner is provided for each of the photosensitive drums in the tandem-type color copier, the sensitivity of the AIDC sensors may vary. In this case, even when the AIDC sensors respectively detects AIDC patterns formed in the same density, the AIDC sensors may output different detection values.
As one example, when the sensitivity of the AIDC sensors for Y and K is low, each density of the yellow and black AIDC patterns is detected lower than the actual density. In this case, these density of yellow and black are corrected in a way that these colors become darker than they should. FIG. 3 shows gradation characteristics of C, M, Y, and K obtained as a result of the incorrect density adjustment. As shown in FIG. 3, yellow and black colors are enhanced. The color of the formed image is out of balance due to the enhanced these colors.
In particular, when a color to be reproduced belongs to a gray area, it is important to reproduce the color in the correct color balance. Variations in the gradation characteristics of toner colors lead to deterioration in the color reproduction.